Tensile testing apparatus

ABSTRACT

A tensile testing apparatus includes an intermittently driven conveyor containing cradles of predetermined positioning shoulder configuration at one end shaped to receive a sample to be tested and to orient the sample transversely to the conveyor feed direction. The conveyor includes a straight line portion for arranging the samples in order before the testing apparatus with their identity tags visible. The sample is passed through apparatus for applying two bench mark stripes in predetermined spaced orientation and subsequently into a position for alignment with the probe of a thickness gauge and thereafter into alignment with a pair of clamping jaws of a tensile stressing device which engage opposite ends of the sample in directions transverse to the conveyor feeding direction. The bench mark stripers apply a pair of side-by-side contrasting bench marks (white and black stripes) in an arrangement for photoscanning irrespective of the color of the material being tested. The clamping jaw mechanism includes means for the controlled outward movement of one of the clamping jaws along with the movement of two photoscanners which are oriented to track the respective bench marks. The stress and strain conditions sensed by the photoscanners and the movable jaws are transmitted to recording means such as a computer along with the measurement of the thickness. The mechanism includes means for removing the tested material from the clamping jaws such as by an air blast and for returning the clamping jaws to an original position along with the resetting of the photoscanners to put the parts in a condition for operation on the next sample moved forward by the conveyors.

- United States Patent [191 Ryckman et al.

[ Nov. 4,1975

[54] TENSlLE TESTING APPARATUS [75] Inventors: George E. Ryckman, EastLiverpool,

Ohio; Walter J. Budd, Waterbury, Conn.

[73] Assignee: Uniroyal, Inc, New York, NY.

[22] Filed: Jan. 21, 1974 [21] Appl. No.: 435,002

Related US. Application Data [62] Division of Ser. No 213,265, Dec. 29,1971, Pat. No.

[52] US. Cl. 73/103 [51] Int. Cl. G01N 3/08 [58] Field of Search 73/103,95; 269/25 [56] References Cited UNITED STATES PATENTS 3,386,283 6/1968Holmes et al 73/103 3,487,680 1/1970 Eichenbrenner et al. 73/1033,721,119 3/1973 Strimel 73/103 X Primary Examiner-Jerry W. MyracleAttorney, Agent, or FirmNorbert P. Holler, Esq.

[57] ABSTRACT A tensile testing apparatus includes an intermittentlydriven conveyor containing cradles of predetermined positioning shoulderconfiguration at one end shaped to receive a sample to be tested and toorient the sample transversely to the conveyor feed direction. Theconveyor includes a straight line portion for arranging the samples inorder before the testing apparatus with their identity tags visible. Thesample is passed through apparatus for applying two bench mark stripesin predetermined spaced orientation and subsequently into a position foralignment with the probe of a thickness gauge and thereafter intoalignment with a pair of clamping jaws of a tensile stressing devicewhich engage opposite ends of the sample in directions transverse to theconveyor feeding direction. The bench mark stripers apply a pair ofside-by-side contrasting bench marks (white and black stripes) in anarrangement for photoscanning irrespective of the color of the materialbeing tested. The clamping jaw mechanism includes means for thecontrolled outward movement of one of the clamping jaws along with themovement of two photoscanners which are oriented to track the respectivebench marks. The stress and strain conditions sensed by thephotoscanners and the movable jaws are transmitted to recording meanssuch as a computer along with the measurement of the thickness. Themechanism includes means for removing the tested material from theclamping jaws such as by an air blast and for returning the clampingjaws to an original position along with the resetting of thephotoscanners to put the parts in a condition for operation on the nextsample moved forward by the conveyors.

4 Claims, 28 Drawing Figures IIIII/ US. Patent N0v.4, 1975 Sheet2of 153,916,681

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0 Close: Jews TENSILE TESTING APPARATUS This is a division ofapplication Ser. No. 213,265 filed Dec. 29, 1971, now U.S. Pat. No.3,803,907.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to general to the construction of tensile testing apparatus, toa method of continuously testing a series of samples and, in particular,to a new and useful apparatus particularly for testing materials havinga high degree of stretch, such as rubber; and which includes means fororienting the samples 'to be tested in a row on a conveyor which ismoved successively into association with means for applying bench marksto the sample, for testing the sample for thickness and for positioningthe sample into association with clamping jaws for effecting the tensiletesting while the bench marks are continuously observed by movablescanners.

2. Description of the Prior Art At the present time, tensile testingmachines are known, which are capable of testing materials displaying alarge amount of stretch, such as cured rubber. Devices of this natureinclude improved automatic apparatus for tensile testing but they havethe disadvantage that they have either no me ans or insufficient meansfor continuously handling a plurality of samples and for presenting themone after the other to a series of testing or preparation stations.

SUMMARY OF THE INVENTION The present invention is an improvement overthe prior art particularly in respect to an improved tensile testingmachine having a supporting cradle for the materials to be tested whichhold the samples in a particular orientation so that they can beadvanced rapidly through the preparation and testing stages withoutdifficulty and also to a method of testing a series of samples. Thetesting equipment of the invention advantageously includes acontinuously operable conveyor with attached individual cradles, shapedto orient the specimens in a particular manner such that neck portionsof the samples extend transversely between the clamping jaws and widenedportions are in a position for easy engagement by the clamping jaws ofthe testing equipment. For this purpose, the cradles include a shoulderconfiguration providing a means for accurately centering and positioningeach sample. The conveyor includes a horizontal portion so that thesamples to be tested may be arranged in a row in visible view with theirmarking indications always discernible prior to their actual testing.

In a preferred form of the invention, the individual samples to betested are arranged in the various cradles defined on an endless chainconveyor, and they are advanced along a horizontal path first intoassociation with an apparatus for automatically applying two bench markstripes thereon which include contrasting areas providing demarcationlines for the subsequent photoscanning of each. The conveyor is thenmoved intermittently to advance the specimens to be tested first intoassociation with a thickness gauge which has a novel rise and fallbottom anvil which moves upwardly through a defined opening of thecarrying cradles when a cradle is aligned therewith. It cooperates withan op- 2 positely arranged gauge member to provide desired thicknesstesting information.

The conveyor is constructed to move between tensile gripping membersarranged on respective sides thereof and to position the sample to betested precisely for engagement by the automatic operation of thetesting jaws. One of the jaws is arranged to engage the testing materialand to move outwardly during the testing procedure, and a scannertransport mechanism is provided to effect a timed corresponding movementof photoscanners associated with each bench mark. The informationobtained from the jaw movement and the applied tensile force on thespecimen as well as the information obtained from photoscanner movementis immediately transmitted to a computer which has previously receivedinformation from the thickness gauge. The apparatus also includes meansfor discharging each sample from the jaws, after the testing isfinished, and for returning and resetting the photoscanners and jawsautomatically.

Accordingly, it is an object of the invention to pro vide a device forpositioning samples for tensile testing which includes a cradle having ashoulder formation for accurately orienting the sample such that widerend portions can be easily gripped by gripping jaws of a tensile testingapparatus.

A further object of the invention is to provide a conveyor system forautomatically applying bench marks in accurate positions on samples tobe tested by tensile testing equipment.

A further object is to provide a method of tensile testing a series ofsamples comprising moving a series of articles together through avisible feed path to present each one in succession first into a markingstation, applying two spaced bench marks to the samples at the markingstation, and feeding each sample in succession to a tensile station andsubjecting the sample to tension while the position of the bench marksare recorded.

A further object of the invention is to provide a conveyor fordisplaying a plurality of samples to be tested in a row for subsequentintermittent movement into association first with bench mark applyingmeans, and then into association with means for obtaining a thicknessvalue of the sample and for subsequent movement into association withclamping elements for tensile testing while the bench marks are scannedby photoscanners.

A further object of the invention is to provide a tensile testingapparatus which includes means for accurately positioning samples to betested automatically into association with clamping jaws of a tensiletester and for moving photocell scanners in a manner to correspond tothe location of two spaced bench marks applied to the testing samplesand to improved apparatus for releasing the sample after testing and forreturning and resetting the photoscanners.

A further object of the invention is to provide a device for testingspecimens for tensile characteristics which is simple in design, ruggedin construction, and economical to manufacture.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference should be had to the accompanying drawing and descriptivematter in which there is illustrated a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 5 is a section taken along theline 5-5 of FIG.

FIG. 6 is a section taken along the line 66 of FIG.

FIG. 7 is a section taken along the line 7-7 of FIG.

FIG. 8 is a section taken along the line 8-8 of FIG.

FIG. 9 is a section taken along the line 9-9 of FIG.

FIG. 10 is a section taken along the line 10-10 of FIG. 9;

FIG. 11 is a section taken along the line 11-11 of FIG. 9;

FIG. 12 is an enlarged plan view of the bench marking device;

FIG. 13 is a section taken along the line 13-13 of FIG. 12;

FIG. 14 is a section taken along the line 14-14 of I FIG. 13;

FIG. 15 is a partial plan view indicating the bench markings providedfor the guidance of the photocells;

FIG. 16 is a section taken along the line 16-16 of FIG. 7;

FIG. 17 is a section taken along the line 17-17 of FIG. 16;

FIG. 18 is a partial top plan view of the photoscanner positioninginformation device;

FIG. 19 is a section taken along the line 19-19 of FIG. 18;

FIG. 20 is a partial enlarged top plan view of the cradle holder andclamping jaws for the sample being tested;

FIG. 21 is a section taken along the line 21-21 of FIG. 20;

FIG. 22 is a section taken along the line 22-22 of FIG. 21;

FIG. 23 is a partial bottom view of a clamping jaw shown in FIG. 20;

FIG. 24 is a perspective schematic view of the thickness measuringdevice;

FIG. 25 is a schematic diagram of the pneumatic controls;

FIGS. 26A, B and C are schematic electrical diagrams for the operatingcircuits.

GENERAL DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawingin particular, the invention embodied therein comprises a first housingpart 10 at one end having a supporting pedestal 12, an intermediatehousing part 14 and a tensile testing section housing part 16. As shownin FIGS. 1 to 3, 7 and 10, the apparatus includes conveyor means,generally designated 18, which is movable from the first end 10successively into association with bench marking means, generallydesignated 20, thickness measuring and recording means, generallydesignated 22, photocell scanning means, generally designated 24, and atensile gripping mechanism, generally designated 26 (See also FIGS. 20and 22). The apparatus also includes an information 'card punch machine,generally designated 28, and a computer or data acquisition andformatting unit, generally designated 30. The card punching unit 28 andthe computer 30 are connected electrically for receiving information inrespect to the thickness measurement unit 22, the gripping mechanism 26and the photoelectric scanning means 24.

In accordance with a feature of the invention, the conveyor means 18comprises an endless chain conveyor with conveyor links 32, as best seenin FIG. 8, which carry or are formed as part of individual conveyorsupport members or sample cradles, gnerally designated 34. The cradles34 have a flat top 35, as shown in FIG. 20, with shoulders 38 and 40 atone end, and 92 and 93 at the other end, which define a shoulderengaging portion for a sample 42 so that a neck portion 44 is heldtransversely to oppositely arranged movable and fixed clamping jawmembers 46 and 48, respectively. The sample 42 includes end portions 42aand 42b which are widened and are arranged in the cradle by theconfiguration of the cradle such that they are oriented for easyengagement by the clamping jaws 46 and 48.

As shown particularly in FIGS. 9, 10 and 11, the drive for the conveyormeans 18 includes a drive motor 50 which provides intermittent rotativemovement to a conveyor sprocket 52 (FIG. 10). For this purpose, themotor 50 drives through a speed reducer 54 to an output shaft 56, whichin turn drives through a chain 58 and a sprocket 60 to an input shaft 62of an intermittent drive mechanism contained in a housing 61, to producean intermittent drive motion at an output shaft 64 which drives theinput sprocket 52 through a sprocket 66 and a chain 68. The conveyormeans 18 includes an endless chain 69, which, as indicated in FIGS. 10and 7, is guided over sprockets 70 and 72 and over a sprocket (notvisible) back to the sprocket 52. The conveyor includes an upper reach18a, which travels from the front housing part 10 in a horizontal pathdown to the tensile testing housing part 16. The conveyor chain 69 ismade up of link elements with the sample cradles 34, as shown in FIGS.20 to 22.

The samples or specimens to be tested 42 are placed in position on eachsuccessive cradle link 34, and they are moved intermittently intoassociation first with the bench marking mechanism 20. The bench markingmechanism 20 is shown in particular in FIGS. 12 to 15, and it comprisestwo sets of spaced striping wheel sets, generally designated 74 and 76,which are driven from a common motor 78 (FIG. 14). The motor 78 includesa shaft 80 having a drive pulley 82 driving a belt 84 and pulleys 86 and88 on bench marker shafts 90 and 90', respectively.

Each bench mark stripping wheel set 74 and 76 is mounted to overlie thecradle 34 in the location between the shoulders 38 and 40 at one end andshoulder elements 92 and 93 at the opposite end, and they are rotated atsubstantially the same speed and same direction as the movement of theconveyor means 18. The sets 74 and 76 are adjustable above the sampleswhich ride in the cradles 34, and they are continuously driven by themotor 78. The wheel sets are located between the spaced sample pieceswhenever the conveyor is at rest.

The bench marking unit, generally designated 20, includes ink fountains94 and 96' associated with each wheel of each set. The set 74 includes awheel 98 and a wheel 100, which are offset in a lateral direction inrespect to the conveyor feed from corresponding wheels 98' and 100' ofthe set 76. The fountains 94 and 96 of the striper wheels of the firstset 74 contain black paint, while the second fountain set (94' and 96')of striper wheels 98' and 100 of the set 76 contain white paint. The twostriper wheel sets 74 and 76 combine to provide side-by-side black andwhite lines, and the bench marks are the separation line between theblack and white lines. FIG. shows a sample of the white bench marks 102and the black laterally adjacent marks 104 with the direction of feedbeing indicated by arrow 106.

In some instances, after the bench marks are applied, it is desired tomove the conveyor along a path bringing it into association withphotoelectric cell scanners (not shown) for the purpose of checking thepositioning of the bench marks. However, in the present instance, theapparatus is shown in an arrangement in which, after the conveyor 18 hasmoved the sample through the bench marking apparatus 20, then moves thesample into association with the thickness measuring means, generallydesignated 22.

The thickness measuring means 22, as shown in connection with FIGS. 7,l6 and 17, includes an anvil portion 110 which projects through anopening 112 of the cradle 34 to engage the underside of the sample 42when the cradle 34 is oriented in a thickness testing position. Theanvil 110 is slidable in a guide 114, which is affixed to a socalledwhiffle tree or crossbar 116 having respective outer ends connected topiston rod members 118 and 120, respectively, of air cylinders 122 and124. The air cylinders 122 and 124 are carried on bracket members 126,which are secured to a support plate 128, which is affixed to theunderside of a conveyor frame 130. Air pressure lines 130 and 132 foractuating each piston rod 118 and 120 to lift the anvil 110 upwardly fortesting purposes are controlled by a cam control mechanism, generallydesignated 134,.as shown in FIG. 10, which includes a cam 136 carried onthe shaft 62, and which actuates a lever member 138 to actuate a valve140 for supplying air pressure to the cylinders 122 and 124.

The thickness testing means 22, as seen in FIGS. 7

and 16 also includes a gauge sensing member 142,

l which overlies the sample 42 and is part of a commercial thicknessgauge 144 having an indicator potentiometer 146. The indicatorpotentiometer 146 is connected to the analog circuit, which in turn isconnected to load cell 266 and computer 30, as shown in FIG. 24.

The air cylinders 122 and 124 have compression springs, which tend tourge the anvil 110 down to a return position and to permit passage ofthe cradles 34 until testing is to take place. The testing is initiatedin the proper timed sequence by the actuating of the air valves 140shown in FIG. 10. The thickness is displayed on the dial indicatorportion of the potentiometer 146.

After the thickness is tested, the conveyor means 18 advances thespecimen into association with the photocell scanning means 24. Thephotocell scanning apparatus comprises two photocell scanners 148 and150,

which are mounted on respective carriages 148a and 150a, as shown, forexample, in FIGS. 7, 4 and 6. Each scanner set 148 and 150 includes alight producing device or lamp 152 and a photocell 154, which arearranged so that the lamp shines a brightly lighted line on the sample42 and the photocell 154 is simultaneously focused on the same spot onthe sample.

A scanner carriage 150a, carrying scanner set 150, is secured to anendless control chain 156, as indicated in FIG. 4, which is displayedbackwardly and forwardly in a straight line during carriage movement.Guide rollers 158 of the carriage are movable along a guide track 160.The carriage 148a carries scanner set 148, and it includes sprocketelements 162 and 164 which engage the chain 156. The sprocket chain 156is guided around an idler sprocket 165 and a sprocket 166 at theopposite end, and it includes a spring section 168 to take up any slack.

Each carriage 148a and 150a also carries a frictional drive wheel 170and 172, respectively, which, as shown in FIG. 6, engages with a levermember 173 when the associated photocell carriage 148a and 150a is to beadvanced during the actual tensile testing of the specimen. A solenoid174 is positioned to pivot the lever about its pivot 177 into an engagedposition when the testing is to take place. When the testing is to bestopped, or during the time of the testing procedure that the scannersare not to be moved, the lever member 173 is permitted to drop back intoits non-actuated position by the action of a spring 175. Each carriage148 and 150 includes a hub portion l48b and 150b, which is slidable on arod member 176 during the time in which it is moving.

A two-level voltage signal from the photocell of each scanner is fed toan amplifier, which has a two-position switching output. The contacts ofthe output switch C1 and NCl energize the solenoid 174. The switching istwo-position. When the photocell 154 sees a dark portion of the benchmark, on the sample, the contacts C1 and NCl close to energize theclutch solenoid 174. Two identical systems are used for the photocellscanners 148 and 150.

The photocell drive comprises a driving motor 178 which drives asprocket 180 and a sprocket chain 182. The chain 182 is guided over aguide sprocket 184 and sprockets 186 and 188 carried on the respectivecarriages 148a and 150a. Rotational movement imparted into the shaft 190of each associated carriage sprocket 186 and 188, as shown in FIG. 16,is transmitted into a shifting movement of the associated carriages 148aand 1500 only when the lever 173 is set by the solenoid 174, and thereis frictional engagement by the frictional drive wheel 172 with thelever 173.

As shown in FIGS. 18 and 19, the sprocket 164 which is connected withthe chain 156 drives a shaft 192, which carries a radially extendingfinger 194, which is located between upright drive detents 196 and 198to provide a lost motion drive for a position sensing disc or encoderdisc 200. The disc 200 is carried on a hub portion 202, which rotateswith the disc around the shaft 192. The motion of rotation is hinderedby a biasing finger 204 which extends outwardly from a fixed holder 206and engages within the recesses 208 of a detent disc 2 10. The finger204 carries a roller 212, which fits into the detent recesses. The disc200 carries a plurality of openings 214 which are moved by the action ofthe finger 194 via the lost motion detents 196 or 198 progressively pastphotocell scanning means 216 which 7 includes a light 218 and aphotocell 219. The detent arm 204 limits any over travel and providesthe precise indexing of the openings 214 in proportion to the movementof the scanner carriages 148a and 150a and this information is fed photooptically to the computer 30.

During the time in which the photocell scanning means 24 is moving inoperative association with the bench marks of the specimen, the specimenis subjected to tensile testing which is effected by the engagement ofthe specimen 42 with the tensile gripping means 26. The tensile grippingmeans 26 includes the movable jaw member 46 and the fixed jaw member 48,each of which includes a base portion 216, as shown in FIGS. 20 to 23,having an upstanding pivot part 2160 and with a lever part 220, which ispivotal on the pivot part 216a and includes a jaw portion 220a, whichengages with a jaw portion 2160 of the fixed part when the sample 42 isclamped. The clamping of each jaw is effected automatically by directinggas or air pressure through a connecting line 222 to urge a piston 224upwardly. The piston 224 includes an upwardly extending piston rod 226which is articulated to the lever, the piston rod for this purpose beingprovided with a through bore 228 into which an end 220b of the leverextends. Upward movement of the piston 224 causes the pivotal upwardmovement of the end portion 220b of the lever and the engagement of thejaws. The fixed or lower jaw part 216 includes a pressure conduit 230for applying a gas or air blast in the vicinity of the clamping jaws,which acts on a recess within a serrated base of the jaws. The upperpart 220 also includes a pressure conduit 232 for the same purpose. Thefixed part 216 carries a conduit 234 with gas or air under pressure toprovide an air blast for releasing the sample from the gripper. Air isalso supplied through a conduit 222 for gripping the sample.

The fixed jaw, generally designated 48, is attached to the machine frameand the movable jaw is secured to a puller head 240, as shown in FIG. 4.The conduits 230, 232 and 234 lead to an external timed air pressuresource. The normal position of each of the gripping assemblies 46 and 48is with the top jaw open; air pressure on the bottom side of the piston224 is vented to the atmosphere and the pressure is applied on the topside through conduit 226. When the sample is to be gripped, the air inthe conduit 236 is released and the conduit 222 is pressurized. When thesample is released, the air in the conduit 222 is released and theconduit 236 is pressurized. The sample is released from the jaw faces bydelivering a timed pulse of high pressure air into the conduits 230 and232 from an external air supply. Thereafter, a horizontal air blast isdirected through conduit 234. As shown in respect to FIGS. and 6, thetensile gripping means 26 includes: separately operable first drivemotor 242 which drives through a clutch 244 to a sprocket 246 for afirst drive speed, a second drive motor 248 which drives through aclutch 250 and a sprocket 252 for a second speed, and a third motor 254which drives a sprocket 256 for the rapid reverse movement. Sprockets246, 252 and 256 are all connected to drive a chain 258 to separatelyrotate a driving sprocket 260 for driving a puller head assembly,generally designated 262, for the movable aw.

The puller head assembly 262 (FIG. 5) comprises a nut which is engagedon a screw 264 and which moves backwardly and forwardly upon rotation ofthe screw which is effected by the sprocket 260 affixed to one endthereof. The puller assembly 262 carries an arm 266 with a stop 268,which is located to engage a sleeve 150b, which is slidable on a rod 176and carries the scanner carriage 150a. By means of this engagement, thestop 268 returns the scanner carriage 150a to a starting position whenthe puller assembly 262 is returned from the maximum end run positionshown in solid lines in FIG. 5 to the dotted lines starting positionunder the driving action of the reversing motor 254 which provides arapid return. The sleeve 150b of the photoscanner carriage 150a willabut against the stop 268 at the zero position of the puller assembly262. As the pull proceeds and the bench mark stripes move apart, thescanner means 24 associated with the carriage 150a follows the leftstripe while the scanner means associated with the carriage 148a followsthe right stripe of the bench mark. As the two scanner heads move apart,the chain 156 (FIG. 4) rotates the sprocket 164 thereby passing theholes 214 of the encoded disc 200 (See FIGS. 18 and 19) successively infront of the photocell 220. Each hole equals 1/ 10 of an inch and eachhole passes a photo optical pulse to the appropriate register of theunit 30 shown in FIGS. 1 and 2.

In the schematic drawing shown in FIG. 24, the potentiometer 146 isshown connected electrically to a v AC source and to a load cell 266having a calibrating attachment 268. The connection of the load cell andthe potentiometer 146 to the computer 30 is also shown.

The pneumatic circuit for the gripping jaws for the movable jaw 46 andthe fixed jaw 48 is shown in FIG. 25. The system indicated shows thevarious solenoid valves 270, 272, 274, 276, 278 and to the air pressurestorage means 284.

FIGS. 26A, 26B and 26C show the electrical diagrams for the variousoperating parts of the overall device.

As shown in FIG. 9, the input shaft 62 of the intermittent drivingmechanism carries a cam 286 which engages and trips a switch 288 atconveyor zero position.

The operation of the device is as follows:

The samples 42 are loaded into the cradles 34 and the bench markstripers are loaded with the respective white and black inks. Thecomputer or DAF unit 30 is turned on and an operator presses the manualstart button 300, as shown in FIG. 26A. The timer T1 starts and carriesout the following:

The lock for the data in the indicator of the thickness gauge 144 isreleased.

The gauge probe 142 is moved downwardly, and the anvil 110 is movedupwardly to measure the thickness. The lock on the indicator 146 isagain applied. The motor 50 is started, and the input shaft 62 causesthe cam 136 to leave the roller 138 of the air valve 140. The air isexhausted from the valve 15. The anvil 110 is moved downwardly throughthe opening of the chain to clear the movement of the chain.

The conveyor then moves forward one cradle length, and the cam 136 againactuates the valve 140 thereby changing from a vented condition of thevalve to a pressurized condition the cause the pressure in the cylinders124 and 126 of the anvil 110 to move the anvil upwardly pressing thewhiffle tree 116 against the end stop and thereby obtaining a positiveand precisely repeatable bottom anvil position. The gauge indicator 146then displays the sample thickness and the potenti-

1. A gripping device for gripping the respective ends of a sample to betesteD for tensile strength, comprising a stationary jaw and a movablejaw positioned for gripping engagement with the opposite ends of thesample, each of said jaws including a lower fixed part defining a fixedjaw member over which the respective end of the sample is placed and anupper movable part defining a movable jaw member for engaging therespective end of the sample from above, said upper movable part being alever pivotally mounted on said lower fixed part, fluid pressureoperated piston and cylinder means connected to the associated lever toone side of the pivot location thereof for moving that lever so as todisplace the respective movable jaw member toward and away from theassociated fixed jaw member, and means for conducting pressurized airinto the space between each movable jaw member and the associated fixedjaw member for separating the sample from the faces of said jaw memberswhen the same are moved apart.
 2. A gripping device according to claim1, wherein said fluid pressure operated piston and cylinder means foreach jaw comprises a hollow cylinder defined in said fixed part of thatjaw, a piston reciprocally slidable in said cylinder, and a piston rodextending upwardly from said piston and articulated to said lever.
 3. Agripping device according to claim 2, wherein said lever of each of saidjaws is pivoted intermediate its ends, the associated movable jaw memberis defined at one end of that lever, and the associated piston rod has atransverse opening therein receiving the other end of that lever.
 4. Agripping device according to claim 1, further comprising means fordirecting a respective blast of pressurized air generally horizontallyinto the space between said fixed and movable jaw members of each ofsaid jaws for expelling the sample from said jaws after the opening ofthe latter and the release of the sample from said jaw members by saidfirst-named pressurized air upon completion of the tensile testing.